


Size Switch

by SoloShikigami



Category: Undertale (Video Game)
Genre: Body Worship, Multi, Praise Kink, Size Difference, Underfell Sans (Undertale)
Language: English
Status: Completed
Published: 2020-05-02
Updated: 2020-05-02
Packaged: 2021-03-02 03:21:39
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,572
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23964544
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Red liked the fact that he was bigger than his boyfriend, Sans - it meant that he could protect him, it usually lead him to leading int he bedroom, Red was the one more in control. Now due to their science shenanigans, the roles are switched. Sans will show Red that it's okay to be vulnerable, and that he loves him no matter what his size is.
Relationships: Sans/Sans (Undertale)
Comments: 5
Kudos: 111





	Size Switch

**Author's Note:**

  * For [silverryu25](https://archiveofourown.org/users/silverryu25/gifts).



> HAPPY BIRTHDAY RYU YOU CUTIE PIE YOU!

Red and Sans were very much alike in many ways at first sight. Besides a couple of cracks and the sharp teeth on Red, it was hard to even tell which was taller. But standing back to back (and after being yelled at to not slouch), Red had quite a few inches on Sans, and he never let him forget it.

Even in the bedroom, where Red also had a few inches on Sans (though of course, magic manipulation and what-have-you).

“One of these days, Red, all that teasing is gonna bite you in the ass,” Sans often warned him, and of course, Red only brushed it off.

Until now, when Red was staring up at Sans who had him nearly pinned against the wall.

“Aw, relax, Red, you know I’m not gonna hurt ya,” Sans said with a chuckle.

“I know that, _bonehead_ ,” Red snapped, anger being his cover for nervousness.

“Red, babe, seriously, relax,” Sans said, backing off and looking down at himself.

They had been messing around with another machine when there was a miscalculation and it zapped Sans into growing. He was now easily twice the size of Red, and he was very interested in throwing his new weight around.

The rest of the day and evening was filled with teasing, not only from Sans, but from both of their brothers as well as their friends. Every now and then, Sans would touch a spot that he knew was sensitive on Red, make a lewd suggestion, keeping Red a little off his usual guard to the point that by the time they retired to their rooms for the night, Red was completely flustered.

“Yeah, yeah,” Red mumbled, not moving from the wall and making a point of not looking at Sans.

“I’m sorry, did I go too far?” Sans asked.

“Naw, naw, I’m cool,” Red muttered, shoving his hands into his pockets.

“Who do you think you’re kidding?” Sans crossed his arms.

Red was already blushing a little, but now, it was nearly in full force.

“It’s weird, y’know, you bein’ bigger th’n me now,” Red mumbled.

Sans looked down at Red, and he knew him well enough, and could feel that there was something more.

“That might be true, but there’s something else, sweetheart,” Sans said, sitting on the floor and patting his lap for Red.

Red glanced over at him and shook his head.

“Come on, Red, out with it, what’s the matter?” Sans asked.

“Nothin’!”

“Red!”

“Wha’ the ‘ell d’y’want me for now, huh?” Red snapped.

Sans blinked, then laughed.

“What’s that supposed to mean?” Sans asked. “Red, come on, you’re being silly.”

“Really now, why would a big guy like you want a shrimp like me?”

“Because I love you, dummy, now get over here or I’ll grab you.”

Red sighed and slowly shuffled over to Sans, and Sans grabbed him around his middle and dragged him into his lap, nuzzling him.

“Talk to me, why would the fact that I’m bigger make me want you any less?” Sans asked once he got Red smiling a little again.

Red sighed, relaxing against Sans a little, knowing that he had better confess.

“Well, b’fore, I was the big guy, so, I could like, protect ya, be good to ya, please ya,” Red said, his voice lowering as he went on. “An’ now, well, what am I gonna do fer ya now?”

“Oh, so you think I’m dating you and sleeping with you just because you’re bigger? Man, oh man, do you have it all wrong,” Sans murmured into the side of Red’s skull, pulling him in closer, hugging him firmly. “It’s nice an’ all, Red, but we’re not in your universe, we’re not in danger, so I don’t need you to protect me. At least, I can take care of you as well as you can take care of me.”

He kissed the side of Red’s face, his hands beginning to wander over his soft clothes. Red shivered under the attention and after a little while, began to squirm.

“Also, I believe we can please each other no matter our size,” Sans said, his touches growing more sensuous. “And I’d really like to please you and see what I can do with this big ol’ bag of bones.”

Red let out a soft moan as Sans nibbled on his neck, one hand slipping under his shirt. Red wanted to know what Sans had in mind, he wanted to hand over it all, but still, there was something holding him back.

“S-Sans,” Red stammered a little. “Y-you know I’m not e’sactly pretty underneath.”

“Shh,” Sans whispered in his ear canal. “None of that, you know I still think you’re amazing. I want to show you, I want to please you, but only if you’ll let me. You know how I feel about this, I won’t force you.”

“I know,” Red sighed, leaning into Sans. “Just, uhm, be careful?”

“You know I will, and of course, we can stop at any time,” Sans said, kissing his temple. “So, that is a yes?”

“It’s a yes.”

Sans smiled, helping Red to turn so he could kiss him properly, and Red leaning into it, trembling a little at the thought of the control he was giving up, but knowing that there wasn’t anyone he would ever be safer with. Despite that, he couldn’t help the slight squeak of surprise when Sans scooped him up and brought him over to the bed, carefully setting him on the edge and kneeling on the floor, though even then he was still looking down at Red.

“This is so weird,” Red muttered as Sans leaned his forehead on his.

“A little, yeah,” Sans said with a grin. “But I think I could get used to it.”

Red gave him a small scowl, but it was soon replaced with an uncertain look as Sans began slipping off Red’s socks, carefully rubbing his feet with his big hands. He leaned in to kiss Red as he pushed the jacket off of his shoulders, Red helping as he pulled his arms out of the sleeves and the jacket was tossed aside, Sans’ soon joining it after. Red looked up at Sans, his eyes a little wide, and Sans grinned, slipping off his t-shirt.

“Talk about big-boned,” Red murmured, his hands gently gliding along Sans’ ribs. “They’re still beautiful.”

“Yeah, yours are, too.”

“Nah.”

“I said they are.”

Sans kissed Red before he could protest again, slipping the shirt up, his hands making an easy pass over Red’s scarred ribcages, the palms of his hands feeling out every crack, dip, and divot, breaking the kiss so he could pull the shirt over Red’s head and tossing it aside to join their other clothes.

“Stars, I could spend hours studying you,” Sans said, gently pushing Red back to lie on the bed.

Red mumbled something under his breath, looking away, and his hands slowly moving to cover himself, but before he could, Sans was hovering over him, bending to place light, gentle kisses on all of his imperfections.

“Each of them are a part of you,” Sans murmured between kisses. “And I love you, and them, and I am completely smitten by such beauty.”

“C’mon Sans, tha’s a lil’ cheesy ever for y-!”

Red’s protest was cut off by his hands being grabbed (they were trying to push Sans away) and pinned firmly to the mattress on either side of his head as Sans’ tongue bathed over the slice across his sternum.

“I said they’re beautiful, and so are you,” Sans said before returning to worshipping all of the scars across Red’s ribcage with kisses and small licks.

Red tried to resist the way Sans’ attention flushed his body with heat, tried to ignore the way his soul began beating harder and faster, tried to block out the sweet words and soft whispers of his lover praising him and his body. But how could he? How could anyone? Then when Sans got to the waistband of his pants, he moved back up, sliding his body against him, pressing his hands easily over his head and kissed him, his tongue pressing into his mouth, keeping the pattern of reaching every tiny vulnerable spot he knew Red had.

When Sans pulled away, he smiled down at the look on Red’s face; he was all but bright red, panting, his eye-lights hazy, and even somewhat heart-shaped. They very rarely looked hazy ever, much less changed shape, and it just awed Sans even more.

“You are so beautiful,” Sans murmured.

Red whimpered, wriggling under him. Sans moved against him, and Red realized that Sans’ magic had formed, and the way it pressed against him put a slight panic into him.

“S-Sans,” he stammered, his eyes wandering down.

“Shh, it’ll be okay,” Sans murmured against his mouth. “I can take care of you.”

He waited a moment, looking into Red’s face again, and while his partner still looked a little unsure, it was clear that Red still trusted him.

“All right,” Red murmured. “I trust you.”

Sans smiled and kissed him, one hand still keeping his hands pinned while the other wandered down his body and under the waistband of his shorts. Red moved his hips into Sans’ touch, and he wriggled against the restraint. Sans started to lighten up, thinking it was too much pressure on Red, when the now-smaller skeleton made a noise of protest.

“No, no, stay, I… uhm, I kinda like it,” Red murmured.

Sans grinned and filed away that bit of knowledge, and he moved so his leg pinned one of Red’s open, giving him more room to coax Red’s magic to coalesce, not that it needed much more coaxing.

Not used to manipulating his magic, Red’s entrance didn’t take the intricate form that Sans usually did, but it didn’t really enter his mind much as all of his focus was purely on making Red feel good. He fingered the small hole carefully - Red’s magic was slippery there and it made it fairly easy for Sans to slip a thick digit into him. Red hissed a little at the slightly unexpected intrusion, which made Sans slow down, kissing him around sensitive spit on his neck and collarbones to distract him from any discomfort. When Red’s hips began moving into him, he pressed more firmly into him.

“Feel good, love?” Sans asked.

Red nodded, not trusting himself to open his mouth.

Sans used the one finger to thrust into Red gently, pressing against the walls, stretching him a little, then he started to carefully press in a second finger. Apparently, he was doing a good job of distracting Red and keeping him aroused, for there was no sign he was in any discomfort or pain. Sans began to carefully scissor his fingers to stretch Red more, which he seemed to take more notice of, as he began to pant and small whines escaped his sharp teeth.

“Sans,” Red moaned.

“Yes?”

“I… please, I… I want more.”

Red’s voice was so shy, Sans found it so cute, but he had to temper his reaction or risk embarrassing Red. So he smiled at him and kissed him gently.

“You are so sexy,” he murmured, nuzzling his neck. “I can’t wait to be inside of you.”

Red made a small whine.

“Of course, if you want me to just keep using my fingers, I can do that.”

“N-no, I want you.”

“Are you sure?”

“Yes.”

Sans started to carefully press in a third finger. Red’s eyes widened, pressing his hips up into it with a slightly desperate edge while groaning.

“Fuck, Sans, please,” Red begged.

“Easy, Red,” Sans said, still trying to be gentle but finding it difficult with Red’s motions.

“No, no, I need you, now, please, Sans, please!”

“Shh, shh, okay, okay,” Sans chuckled, carefully pulling his fingers out so he could shove his shorts down and Red’s. “Damn, it’s really hot to see you like this.”

“Yeah, yeah, sh-shut up.”

Red couldn’t help looking down; Sans had pulled Red’s shorts of his hips, just barely exposing his entrance, and then went to push down his own shorts, his cock springing free from the elastic band. It was bigger than Red expected, and part of him wondered if the prepping he allowed was enough, but another part of him didn’t care and just wanted his lover inside of him _now_.

Sans had let go of Red’s hands, but they stayed where they were. Sans lined himself up with Red, and after giving one more glance up at him for permission, Red whined, moaned out a “please,” and Sans moved in.

Red tried to be quiet, but as Sans sank into him, he couldn’t. He arched and cried out, the sensation of being stretched and filled almost too much for him.

“God damn, you are gorgeous,” Sans moaned. “So tight, so sweet, fuck, Red, I love you so much.”

The orgasm hit Red like a tsunami, his body shook, Sans grunted as Red’s magic sucked him in and squeezed him tighter. He leaned over Red, trying to keep his breathing controlled as the other spasmed under him.

“Fuck, fuck, fuck,” Red chanted.

“Easy there, good boy,” Sans whispered, leaning down to kiss Red to help calm him.

Red gripped onto Sans, kissing him back, and when the kiss broke, he looked up at him.

“Fuck me, Sans, please,” Red moaned.

“As you wish, baby.”

Sans began to thrust gently, as Red was still gripping him, but it didn’t take long for him to loosen again, allowing Sans to pick up the pace. Sans continued to shower Red with kisses and praises, his hands wandering over his body, playing along his scars, riling him up again.

“Red, I’m getting close,” Sans murmured.

“Cum inside me, Sans,” Red whispered back.

It was like a command that Sans had no choice but to obey. He barely even had to chance to move faster, Red’s voice being enough to send him over the edge, spilling his magic inside of Red. Red could feel it all, and the sensation of being filled suddenly overwhelmed him, making him cum again as well as he felt Sans’ hot magic spilling out of him.

“Damn,” Sans panted, leaning over Red, kissing him as he thrust a few more times, loving the feel of their releases surrounding him.

Red whined, kissing Sans back.

“Fuck,” Red groaned as Sans pulled out carefully. “That’s gonna be sore later.”

“I’m sorry,” Sans said. “I’ll take good care of ya.”

“I know.”

Red pulled Sans down for another deep kiss, the two of them sighing as they slowly came down from the peaks of their pleasure, cuddling and enjoying each other’s warmth.

“So, uhm,” Red broke the silence after a few moments, seeming to wriggles nervously. “C-can we do that again?”

“Sure,” Sans said.

“Like… now?”

Sans blinked at him.

“And… uhm… mebbe you could… pin me down again?”

Sans blushed a little, but finding that the request made him happy; he knew that Red trusted him, but this was showing a new, deeper level of trust and it made his soul soar.

“Anything for you, sweetheart,” Sans said.

“I… this is stupid, but, if you stay this way, I, uhm, can’t say I’d be unhappy,” Red added, blushing furiously. “Either way, I love you, Sans.”

“Heh, either way, love you, too,” Sans said.

**Author's Note:**

> Like what you read? Please check out my social media for more fun!  
> [ My Tumblr](https://soloshikigami.tumblr.com/)  
> [ My Twitter](https://twitter.com/SoloShikigami)


End file.
